The present invention generally relates to a level shifter spare cell and, more particularly, to a method for configuring the level shifter spare cell for shifting levels among power domains.
In integrated circuits having different power domains, level shifters are needed to convert signal power levels for signal transitions between the different power domains. Spare cells are provided in integrated circuits for correcting design errors or malfunctions detected after a mask set of an integrated circuit has been formed (i.e., post-mask). For malfunctions between power domains, normally multiple level shifter spare cells are provided corresponding to each possible path from one power domain to another. For example, in a system having two power domains, Vdd1 and Vdd2, at least two level shifter spare cells are required, with one of the spare cells for the connection from Vdd1 to Vdd2, and the other for the connection from Vdd2 to Vdd1. Accordingly, for a system having ‘n’ power domains, the number of the level shifter spare cells is at least n*(n−1).
FIG. 1 is a schematic diagram of an integrated circuit or system having multiple power domains and corresponding connections of spare cells between the multiple power domains. The system 100 includes four power domains, respectively domain a, domain b, domain c, and domain d. Arrows between two of the power domains represent the possible paths for level shifter spare cells to be coupled. Operations of the level shifters have directions, which means that for the conversion of power levels between domain a and domain b, two level shifters are used, respectively from domain a to domain b, and from domain b to domain a. In the system 100, at least twelve spare cells are used to cover the possible paths between the four power domains.
Spare cells usually are distributed along boundaries of the power domains, which normally are considered as critical areas because of the high density and high occupancy of circuit devices. On the other hand, as compared with normal combinational logic devices, level shifter cells are relatively large in size and accordingly consume more space in the critical areas. Further, if more than one level shifter is used for the normal operation in one of the paths, then the same number of level shifter spare cells must be coupled in the path, which adds to the circuit occupancy and density in these critical areas.
It would be advantageous to have flexible spare cells that require less area to save space in critical areas.